共查询到20条相似文献,搜索用时 555 毫秒
1.
2.
物理层多包接收技术的发展给利用物理层多包接收能力的媒体接入控制(MAC)协议的设计带来了挑战。IEEE802.11 DCF是目前WIAN最成熟的分布式MAC协议之一,对其在多包接收模型下进行性能改善将有很大的应用价值。在物理层具有多包接收能力的基础上,提出了一种改进的802.11 DCF协议,并将该协议应用于现有的基于802.11 DCF的多包接收MAC算法(MDCF),理论分析和NS-2仿真实验结果表明,该算法与IEEE 802.11 DCF和MDCF相比,在网络吞吐量和时延性能方面有很大的改善。 相似文献
3.
新的改进IEEE 802.11 DCF性能的退避机制 总被引:1,自引:1,他引:0
分布式协调功能DCF是IEEE802.11标准最基本的媒体接入方法,它的核心是载波检测多址接入/冲突避免(CSMA/CA)机制,通过退避算法,减少碰撞的概率。提出了一种新的退避机制改进IEEE802.11DCF饱和吞吐量性能,建立了三维马尔可夫链网络模型详细研究分析,同时利用NS2对所提出的机制进行仿真,比较了改进后的802.11DCF饱和吞吐量与原802.11DCF的饱和吞吐量的大小,仿真结果证明了算法的准确有效。 相似文献
4.
5.
在考虑节点的物理载波检测范围大于通信范围的情况下,该文分析了多跳Adhoc网络中物理载波检测机制对IEEE 802.11DCF协议公平性的影响。针对载波干扰给IEEE802.11DCF协议带来的严重不公平问题,提出了一种基于冲突和干扰感知的退避(CIAB)算法。仿真证明,该算法能有效地改善IEEE802.11DCF协议的公平性,并且没有引起网络吞吐量的严重下降。 相似文献
6.
该文提出了IEEE802.11DCF差错帧模型的概念,并在详细分析这一模型的基础上,指出了GloMoSim仿真环境中现有的DCF差错帧模型仿真算法存在的问题,然后严格按照DCF差错帧模型中的相关规约改进了该仿真算法。仿真与分析表明,与GloMoSim中现有的仿真算法相比,改进后的仿真算法能正确地模拟IEEE802.11DCF协议处理差错帧的相关规约,为仿真实验提供了更加可靠的结果。 相似文献
7.
8.
9.
10.
在IEEE 802.11b协议中有四种速率用于数据传输,节点可根据信道情况选择传输速率,致使802.11DCF公平的退避机制不再适用.为了让高速节点能更容易的竞争到信道以体现多速率协议的优势,提出了基于拥塞控制的速率自适应机制.竞争信道的节点根据拥塞情况来选择合适的退避窗口.通过NS2软件仿真证明该机制进一步提高了竞争窗口自适机制的性能. 相似文献
11.
In order to diagnose the laser-produced plasmas, a focusing curved crystal spectrometer has been developed for measuring the X-ray lines radiated from a laser-produced plasmas. The design is based on the fact that the ray emitted from a source located at one focus of an ellipse will converge on the other focus by the reflection of the elliptical surface. The focal length and the eccentricity of the ellipse are 1350 mm and 0.9586, respectively. The spectrometer can be used to measure the X- ray lines in the wavelength range of 0.2-0.37 nm, and a LiF crystal (200) (2d = 0.4027 nm) is used as dispersive element covering Bragg angle from 30° to 67.5°. The spectrometer was tested on Shengnang- Ⅱ which can deliver laser energy of 60-80 J/pulse and the laser wavelength is 0.35 μm. Photographs of spectra including the 1 s2p ^1P1-1s^2 ^1S0 resonance line(w), the 1s2p ^3P2-1s^2 1S0 magnetic quadrupole line(x), the 1s2p ^3P1-1 s^2 ^1S0 intercombination lines(y), the 1 s2p ^3S~1-1 s^2 ^1S0 forbidden line(z) in helium-like Ti Ⅹ Ⅺ and the 1 s2s2p ^2P3/2-1 s622s ^2S1/2 line(q) in lithium-like Ti Ⅹ Ⅹhave been recorded with a X-ray CCD camera. The experimental result shows that the wavelength resolution(λ/△ 2) is above 1000 and the elliptical crystal spectrometer is suitable for X-ray spectroscopy. 相似文献
12.
High purity organic-tantalum precursors for thin film ALD TaN were synthesized and characterized.Vapor pressure and thermal stability of these precursors were studied.From the vapor pressure analysis,it was found that TBTEMT has a higher vapor pressure than any other published liquid TaN precursor,including TBTDET,TAITMATA,and IPTDET.Thermal stability of the alkyl groups on the precursors was investigated using a 1H NMR technique.The results indicated that the tertbutylimino group is the most stable group on TBTDET and TBTEMT as compared to the dialkylamido groups.Thermal stability of TaN precursors decreased in the following order:TBTDET > PDMAT > TBTEMT.In conclusion,precursor vapor pressure and thermal stability were tuned by making slight variations in the ligand sphere around the metal center. 相似文献
13.
This paper reviews our recent development of the use of the large-scale pseudopotential method to calculate the electronic structure of semiconductor nanocrystals, such as quantum dots and wires, which often contain tens of thousands of atoms. The calculated size-dependent exciton energies and absorption spectra of quantum dots and wires are in good agreement with experiments. We show that the electronic structure of a nanocrystal can be tuned not only by its size,but also by its shape. Finally,we show that defect properties in quantum dots can be significantly different from those in bulk semiconductors. 相似文献
14.
YUXiao-hua XIANGYu-qun 《半导体技术》2005,30(2):30-32,37
An improving utilization and efficiency of critical equipments in semiconductor wafer fabrication facilities are concerned. Semiconductor manufacturing FAB is one of the most complicated and cost sensitive environments. A good dispatching tool will make big difference in equipment utilization and FAB output as a whole. The equipment in this paper is In-Line DUV Scanner. There are many factors impacting utilization and output on this equipment group. In HMP environment one of the issues is changing of reticule in this area and idle counts due to load unbalance between equipments. Here we'll introduce a rule-based RTD system which aiming at decreasing the number of recipe change and idle counts among a group of scanner equipment in a high-mixed-products FAB. 相似文献
15.
The epi material growth of GaAsSb based DHBTs with InAlAs emitters are investigated using a 4 × 100mm multi-wafer production Riber 49 MBE reactor fully equipped with real-time in-situ sensors including an absorption band edge spectroscope and an optical-based flux monitor. The state-of-the-art hole mobilities are obtained from 100nm thick carbon-doped GaAsSb. A Sb composition variation of less than ± 0.1 atomic percent across a 4 × 100mm platen configuration has been achieved. The large area InAlAs/GaAsSb/InP DHBT device demonstrates excellent DC characteristics,such as BVCEO>6V and a DC current gain of 45 at 1kA/cm2 for an emitter size of 50μm × 50μm. The devices have a 40nm thick GaAsSb base with p-doping of 4. 5 × 1019cm-3 . Devices with an emitter size of 4μm × 30μm have a current gain variation less than 2% across the fully processed 100mm wafer. ft and fmax are over 50GHz,with a power efficiency of 50% ,which are comparable to standard power GaAs HBT results. These results demonstrate the potential application of GaAsSb/InP DHBT for power amplifiers and the feasibility of multi-wafer MBE for mass production of GaAsSb-based HBTs. 相似文献
16.
Tian-hua Xu Feng Tang Wen-cai Jing Hong-xia Zhang Da-gong Ji Chang-song Yu Ge Zhou Yi-mo Zhang 《光电子快报》2008,4(4):292-294
Distributed polarization coupling in polarization-maintaining fibers can be detected by using a white light Michelson interferometer. This technique usually requires that only one polarization mode is excited. However, in practical measurement, the injection polarization direction could not be exactly aligned to one of the principal axes of the PMF, so the influence of the polarization extinction ratio should be considered. Based on the polarization coupling theory, the influence of the incident polarization extinction on the measurement result is evaluated and analyzed, and a method for distributed polarization coupling detection is developed when both two orthogonal eigenmodes are excited. 相似文献
17.
We calculate the Langevin noise sources of self-pulsation laser diodes, analyze the effects of active region noise and saturable-absorption region noise on the power fluctuation as well as period fluctuation, and propose a novel method to restrain the noise effects. A visible SIMULINK model is established to simulate the system, The results indicate that the effects of noise in absorption region can be ignored; that with the increase of DC injecting current, the noise effects enhance power jitter, and nevertheless, the period jitter is decreased; and that with external sinusoidal current modulating the self-pulsation laser diode, the noise-induced power jitter and period jitter can be suppressed greatly. This work is valuable for clock recovery in all-optical network. 相似文献
18.
Large-scale synthesis of single-crystal CdSe nanoribbons is achieved by a modified thermal evaporation method, in which two-step-thermal-evaporation is used to control CdSe sources' evaporation. The synthesized CdSe nanoribbons are usually several micrometers in width, 50 nm in thickness, and tens to several hundred micrometers in length. Studies have shown that high-quality CdSe nanoribbons with regular shapes can be obtained by this method. Room-temperature photolumines-cence indicates that the lasing emission at 710 nm has been observed under optical pumping (266 nm) at power densities of 25-153 kW/cm^2. The full width half maximum (FWHM) of the lasing mode is 0.67 nm 相似文献
19.
TANG Bin JIANG Xing-fang LIU Zhi-min 《光电子快报》2008,4(1):78-80
By using the expansion of the aperture function into a finte sum of complex Gaussian functions, the corresponding analytical expressions of Hermite-cosh-Gaussian beams passing through annular apertured paraxially and symmetrically optical systems written in terms of ABCD matrix were derived, and they could reduce to the cases with squared aperture. In a similar way, the corresponding analytical expressions of cosh-Gaussian beams through annular apertured ABCD matrix were also given. The method could save more calculation time than that by using the diffraction integral formula directly. 相似文献
20.
正With the support of 863 programs,Sugon Information Industry Co.,Ltd.,set up a dawning EB-class storage laboratory to address massive data storage requirements and largescale cloud computing demonstration applications.The Dawning EB-class cloud storage system adopts advanced fault-tolerant architecture,efficient data fault-tolerant algorithms with user authentication and data encryption policies to deal with the"lost""wrong"and"stolen"problems of data for ensuring the reliability and safety of the EB-class storage system in the public network application environments.The Dawning EB-class Storage Laboratory taking advan- 相似文献